In a hot-dip galvannealing treatment step for a steel strip, the steel strip passes through a pre-treatment bath for degreasing, cleaning, or the like and then passes through an annealing furnace and a zinc pot containing molten zinc, then being raised perpendicularly. The raised steel strip is subjected to galvannealing treatment in a galvannealing furnace. The galvannealing furnace includes a heating zone and a cooling zone arranged from the upstream side in a direction in which the steel strip is raised.
That is, the cooling zone of the galvannealing furnace is arranged vertically above the heating zone. Therefore, cooling of the steel strip in the cooling zone is performed using gas cooling or mist cooling so as not to exert an influence, such as dripping water, on an installation arranged vertically below the cooling zone. In particular, it is effective to use mist cooling (mist cooling) which has high cooling capacity in order to improve production capacity. In mist cooling, however, in the case where a large amount of water is sprayed in order to strongly cool the steel strip, temperature unevenness occurs in the width direction of the steel strip. This temperature unevenness causes quality defects, such as wrinkles and zinc powder pick-up.
In view of such a problem, for example, Patent Literature 1 discloses a galvannealing furnace exit-side mist cooling method in which a cooling pattern of a steel strip is adjusted so that temperature deviation in the width direction due to overcooling is suppressed. In Patent Literature 1, in order to suppress cooling variation due to dripping water and make temperature unevenness equal to or less than wrinkle limit temperature unevenness, a steel strip is cooled in a manner that a cooling ratio between a preceding stage and a subsequent stage of a cooling zone is changed so that the subsequent stage is subjected to slow cooling.
Patent Literature 2 discloses a cooling method in a galvannealing treatment process. The method uses either of gas cooling and mist cooling according to cooling load to avoid transition boiling and suppress temperature deviation in the width direction.
Furthermore, Patent Literature 3 discloses a technology of arranging nozzles densely in a center portion in the width direction of a steel strip and providing shutters for blocking the nozzles.
Patent Literature 4 discloses a technology of controlling a tension value and temperature unevenness based on a predetermined relational expression to set a cooling zone exit-side temperature to 240° C. or lower in order to prevent reduction of area and buckling of a steel sheet at the exit side of a mist cooling installation.
Patent Literature 5 discloses a technology of using either of mist cooling and cooling with gas for each zone to avoid a transition boiling region, which causes cooling variation, in order to make an Fe concentration amount in a plating layer appropriate.